Motor speed control



Jan. 3, 1950 C. MOTT EI'AL MOTOR SPEED CONTROL Original Filed April 4, 1944 2 Sheets-Sheet 1 POWER PRE F ER I l AMLIFlER J46 8 33' 8a C 85 81 78 15 1a? 82 l- 4 7a 86 INVENTORS C'hesi'er MotifredFC/zouinard ATTORNEYS Jan. 3, 1950 c. MOTT ETAL 2,493,079

MOTOR SPEED CONTROL Original Filed April 4, 1944 2 Sheets-Sheet 2 77 5 Che 22 EJNJOQ? 22g g 50 A gfrZd g'lz a mam:

ATTORNEYS Patented 3, l9

MOTOR SPEED CONTROL Chester Mott, Evanston, and Alfred F. Chouinard, Chicago, Ill., assignors to National Cylinder Gas Company, Chicago, 11]., a corporation of Delaware Original application April 4, 1944, Serial No.

886,874. Divided and this application February 16, 1944, Serial No. 522,566

'1 Claims. l

The present invention broadly considered relates to variable speed motors, and more par-" be kept constant within close limits by the rheostat or any other known means. With any given rheostat setting, the speed of the motor will vary with fluctuations in load; variations in the amount of current tapped of! the line by other electrical equipment in the plant; warming up of the motor which changes the conductivity of the motor windings; warming up of the lubricating oil, which reduces its viscosity; and various other factors.

A synchronous motor will hold a definite and fixed speed, but that speed is one determined by the builder of the motor (i. e., by the number of poles), and by the frequency of the current available, which is ordinarily 60 cycles, and this speed cannot be altered or modified by the user. There are some exceptions, for instance, where the motor is of the multi-pole type, and certain poles may be cut out of the circuit, but any such cuting in or out of the poles does not permit more than a very small number of speed changes, and does not permit any small preselected variations.

It is possible to operate a synchronous motor at one definite speed and obtain various required output speeds by means of mechanical gearing. Such gearing causes loss of power through friction, and requires considerably more space to accommodate the gearing and the gear shifting mechanism than is required to accommodate the synchronous motor, especially if the range of speeds is large.

One object of the present invention is to provide means for operating a synchronous motor through a wide range of selective speeds, while maintaining any chosen speed at an extremely high degree of constancy.

Another object is to provide means for operating a synchronous motor through a wide range of selective speeds by changing the frequency of the current impressed on said motor.

Another object is to provide means for selectively changing the speed of a motor in a numbar of preselected steps or continuously over a wide range of speeds.

Another object is to provide means for facilitating rapid, easy and exact duplication of a given motor speed.

Various other objects, features and advantages or the invention will be apparent from the following particular description, and from an inspection oi the accompanying drawings, in which:

Fig. 1 is a schematic wiring diagram of a device by means of which the speed of a synchronous motor can be selectively changed.

Fig. 2 is an illustration of a photo-electric means of generating a current of'a predetermined selective frequency which may be used to control the speed of a synchronous motor.

Fig. 3 is a fragmentary plan view 01' a disc, forming part of the device for changing the frequency of the current to the synchronous motor.

Fig. 4 is a fragmentary plan view of another form of disc.

Fig. 5 is a plan of an electro-magnetic means for generating a current of a predetermined selective frequency which may be used to control the speed of a synchronous motor in place of the photo-electric means illustrated in Fig. 2.

Fig. 6 is an edge view of the apparatus shown in Fig. 5, the disc being shown in section.

Fig. 7 is a somewhat diagrammatic section showing a circuit controller forming a part of the electrical system of Fig. 1, and

Fig. 8 is a section taken on the line 8-8 of Fig. 7.

As a feature of the present invention, the motor i6 is of the synchronous type, and the frequency of the current supplied thereto is changed in accordance with the desired motor speed. This change in frequency is eflected by creating periodic current pulsations at selective frequencies, depending on the desired motor speed, and amplifying these pulsations to-a value necessary to operate the synchronous motor 16.

One form of construction for obtaining current pulsations of selective frequencies is shown in Figs. 2 and 3, and comprises a disc 22 which is provided with a plurality of concentric annular series or rows of alternate opaque segments 23, and light transmitting segments 24, and which is driven at a constant speed from a synchronous motor 25 through suitable reduction gearing; the synchronous motor 25 being operated from a powed source A or other suitable alternating rent power.

The light transmitting segments 24 may constitute slots in an opaque disc, or the disc 22 may be of glass or other suitable transparent material, coated or fired with a black pigment, or otherwise rendered opaque at all sections except at the light transmitting segments 24.

A source of light 28 shown in the form of an exciter lamp, and a light reactive device 21 shown in the form of a shielded photo-electric cell are disposed on opposite sides of the disc 22, and are movable in unison radially of the axis of said disc into registry with any selective row of light transmitting segments 24. When one of these segments 24 moves into registry with the line of light from 26, through an aperture to in the shield 3| around the photo-electric cell 21, current is generated in said cell, and when an opaque segment 23 moves into said line of light, the generation of current in said cell stops. As a result, there is generated in the photo-electric cell 21 a pulsating current having a frequency depending on the number of light transmitting segments 24 in the registering row, brought into and out of position per unit of time.

The number of light transmitting segments 24 in each row varies according to the desired speed of the motor ii. For instance the inner row may have ten transparent segments 24 and ten opaque segments 23 alternately arranged. With the disc 22 making two revolutions per second, the frequency obtained by using the inner row would be 20 cycles per second (one transparent and one opaque segment constituting one cycle). The next row may be arranged to generate 22 cycles, the following row 24 cycles, and so on up to the outer row which may be arranged to generate 400 cycles. With this range of frequency available it is possible to vary the speed of themotor it over a 20 to 1 range. The range of frequencies generated by disc 22 would be chosen to cover the necessary range as required by any specific application of the motor it.

Instead of providing well defined alternate opaque and transparent segments 23 and 24, as shown in Fig. 3, it is preferable to make the disc 22a as shown in Fig. 4, with the light transmitting sections 24a graded from absolute transparency to absolute opacity, to produce sinusoidal or similar forms of undulated pulsations.

As far as certain aspects of the present invention are concerned, other means may be provided for obtaining current of selective frequencies For instance, as illustrated in Figs. 5 and 6, alternating current may be induced in a coil wound on a soft iron core 5|, of suitable design. The ends of the core are pointed and disposed opposite to each other on opposite sides of a steel disc 22b having a plurality of annular rows of slots. With the coil opposite one of said rows, current will be induced therein when the solid parts of the disc pass opposite said coil, the induced current diminishing to zero as the slots pass opposite said coil. The coil is so mounted as to be movable radially of the disc, and to a position opposite to any one of the rows of slots.

Also as far as certain aspects of the invention are concerned, a standard type of signal generator well known in the art can be employed to obtain minute current pulsations of selective frequency.

As a feature of the present invention, the change in frequency of the current to the motor It is effected by manually turning handwheel 32 as shown in Fig. 2, until pointer 22 registers with the desired setting on chart II, chart 3! being of a well known type which has been calibrated be- 4 forehand in R. P. M. of motor it, or with other suitable required operating information.

The transmission between this handwheel 32 and the pointer 33 may be of any desired type. The handwheel is shown as secured to an axially fixed lead screw 31 on which there is threaded a nut 38 to which is amxed an arm 39 ca Y B the pointer 33.

In order to correspondingly position the light source 26 and the light reactive device 21 with respect to the pointer 33, there is' also rigidly affixed to nut 38, a yoke 40, with side arms carrying the members 26, 21 and II.

In order -to insure that the members 28 and 21 are stopped in proper alignment with the desired frequency control ring on the disc 22, there is provided an index device which may comprise a cam 42 secured to the lead screw 31 for rotation therewith, and provided with one or more recesses 43. A roller 44 carried on a pivot arm ll is urged against the periphery of the cam 42 by a spring 46. When the roller 44 is in one of the recesses 43, the light 26 and the light reactive device 21 will be in proper alignment with one of the frequency control rows. The operator can tell when this position is reached by the feel of the handwheel 32 as it turns easily into correct position, and greater resistance is encountered in moving the cam out of acorrect position. Obviously, the U-shaped core bar 5| shown in Figs. 5 and 6 could be secured to the nut 28 in place of the yoke 40.

All of the parts shown in Fig. 2 are mounted in a suitable manner, preferably in a casing or housing with the chart 3!, and the pointer is being readily visible from the outside, and the end of the lead screw 31 bearing the handwheel 32 brought to the outside to facilitate selection of the desired speed. To facilitate a clear showing of the parts appearing in Fig. 2, they have been spread apart, and in somewhat different relative positions than they would occupy in commercial practice. Various other operating connections may be employed.

In Fig. l is shown a wiring diagram of a form of electrical system which may be employed for amplifying the minute current pulsations generated in the photo-electric cell 21 to a value necessary to operate the synchronous motor I]. In this system, the main input lines A are connected to a commercial source of alternating current which is usually 60 cycles, volts. When main switch I II is closed, power is available for operation of the device.

Current flows from the left branch of the main A through points 16 and 20, through the motor 25, through points 2| and 'Il, back to the right branch of the main A. The current available at point 2| flows through the filament of the exciter lamp 26 and returns to point 20 on the main. The minute current pulsations generated by the photo-electric cell 21 are delivered to the pre-amplifier C through points 11 and 18.

The current is branched ofi! from point H on one line of the main to points I2 and 18, through various transformers of apower amplifiers B of well known construction, to reduce the voltage to values suitable for operation of the tube filaments in said amplifier, and also to supply power to the tubes in a pre-amplifier C of well known construction in the art. The current then returns to points I4 and I5, and then to point 16 on the return line of the main.

Current from the main A is also delivered through a motor stop and start switch it in power amplifier B and applied to the plates of the various thermionic tubes employed.

With the disc motor 25 revolving the frequency control disc 22, and the exciter lamp 26 focused on the photo-electric cell 21, and the filaments of the various vacuum tubes supplied with their proper voltages, it is only necessary to supply high voltages of direct current to obtain a power output from the power amplifier B, and available hetween points 84 and 85.

The frequency of the current of said power output is determined by the frequency control disc 22. This current flows from point 84 through points 88 and 81, through the running winding 88 of the synchronous motor I0, through point 90, and returns to point 85. There is also a parailel path from the point 84 through points 86 and 81, through contact 8| in the dotted line position shown, through a conductor strip 32, through a contact arm 93, through one of the starting condensers 98, 91, 88, 88, for the motor I8, through the starting winding I00 of the synchronous motor l8, through point 90, and back to point 85. A relay IOI controls the timing closure of the contact 9I', as will be described,

The motor I6 is of the capacitor start type requiring one of the condensers 98, 91, 88, 98 in series with the starting winding I00, as will be fore fully described. When a standard motor of this type is used on a standard commercial frequency, the motor rotor is equipped with a centrifugal throw-out switch in the circuit of the starting winding I00. If a motor has been designed to run at a synchronous speed of 1800 rotor revolutions per minute, then the centrifiigal throw-out switch is so designed as to be opened or thrown out at approximately 1200 rotor revolutions per minute. In the application of this type of capacitor start motor with frequency control, the standard type of centrifugal throwout switch would not besuitable, as the rotor must at times be rotating at synchronous frequencies as low as 600 revolutions per minute, and as high as 12,000 revolutions per minute.

It is common knowledge to those versed in the art that the power generated by the starting winding I00 must be available to bring the motor rotor up to nearly synchronous speed, and the rotating electrical field which determines the synchronous rotating speed of the motor must be allowed to keep the rotor in step at this frequency. It is not important that the starting winding I00 be cut off elecrically just before the motor rotor attains synchronous speed, but it is necessary that said starting winding remain connected long enough to bring the rotor almost up to synchronous speed at the highest frequency. It is also necessary that the starting winding I00 be cut off after the rotor has reached almost synchronous speed to allow the motor to rotate solely by the effect generated by'the running winding 88. To apply the current to the starting winding I00 long enough to bring the rotor up to nearly synmay be as high as 400 cycles. and cutting it oi! electrically at this point, there is provided a synchronous timing motor I03, and a cam switch device I18 operated from said timing motor.

The cam switch device II6 comprises three cams I04, I05, and I08 mounted for rotation in unison on a shaft I01, driven from the timing motor I03 as shown in Figs. '1 and 8. The cam I04 is adapted to operate a switch I08 into the solid line or the dotted line position shown in Figs. 1 and 8, so that when one of said positions is opened, the other is closed. Two other switches I10 and III are operated from the cams I05 and I08 into the solid line or dotted line position showninFig. 1. The three cams I04, I05 and I08 are designed to move the switch I08 to the solid line position from 0 to 10 of rotation during one revolution of the cams; move switch I08 to the dotted line position from 11 to 360; move switch IIO to the dotted line position from 355 to 360; and move the switch I II to the dotted line position from 15 to 225. During the other portions of the timing cycle, switches H0 and III are in the solid line position.

When the motor start and stop switch 80 is closed, current is not only rendered available to the power input transformers through points 8I and 82, as hereinbefore described, but current also flows from point I3I to points I32 and I33, through contact I35 and switch I08 in the solid line position, through the timing motor I03 to rotate said motor, to points I38 and 12, and back to the main line at point 1|. Switch I08 is in the solid line position from 0 to 10 during rotation of the cams, and is moved to the dotted line position between 10 and 11. The switch I08 is of the quick action type, and is so constructed that it is either in the solid line or dotted line position with no intermediate position except during a small fraction of a second required to effect the switch-over. With switch I08 in the dotted line position shown, the current does not have to flow through the motor stop and start switch 80 to continue rotation of the timing motor i03 through the remainder of its timing cycle, but will flow from point 18 on one line of the main A, through point 15, through switch I08 in the dotted line position, through the timing motor M13, through points I35 and 12, to point H on the other line of main A.

It is sometimes necessary to supply power to synchronous motor I6 for only a short interval of time; for instance to move the device it drives to a desired position. This'is accomplished by closing switch 80 to supply power to motor I6, and at the appropriate time, reopen switch 80, thereby cutting off the supply of power to motor IS.

The action of switch I08 allows the motor start switch 80 to be opened to stop the drive motor l8, yet providing current to be supplied to the timiliig motor I03 to return it to its original 0' posit on.

Switch III supplies current to the relay coil IN, and for that purpose is closed from 15 to 65 255 of cam rotation as described, to provide current flow through switch 80, through points BI and I32, through switch III in dotted line position shown, through relay coil I0 I, through points I31, I38, I38 and 12 and to point 1| on the main A The energization of the relay coil I0l causes movement of the contact 9| to the dotted line position shown, and thereby causes fiow of current to the starting winding I00 from the output of the power amplifier B, as already dechronous speed at the highest frequency which scribed.

I The purpose of switch I I0, which is closed from 355 to 360 of cam rotation, is to energize a tim ing relay I40 to open the circuit of the timing motor I08, and thereby allow said motor to rotate only through one timing cycle. When this switch III is closed to the dotted line position shown, current will flow through switch 80, through points I3I, I32 and I33, through relay coil I 40, through point I, through switch I I0, through points I38, I36 and 12, and to point H on the main line. Relay coil I40 will now become energized closing a contact I42 to the dotted line position shown, and opening contact I35 into dotted line position shown. When contact I42 is closed, the current does not have to flow from point I, through switch I I0, through points I38, I36 and 12 and on to point II on the main, but flows directly from point I 4|, through contact I42, through points I31, I38, I38 and 12 and on to H on the main. When contact I35 is open in dotted line position shown, it does not allow current to fiow through switch I08 in the solid line position shown. This switch I08 however is in the dotted line position from 355 to 360 to deliver current to the timing motor I03. Thus the timing motor I03 will continue to rotate until switch I08 is moved to the solid line position. Then at 360, switch I will be moved from the dotted line position to the solid line position, but due to contact I85 being opened by virtue-of the relay coil I40 being energized, the timing motor will come to rest at its normal position, but is ready at any time thereafter to repeat its timing cycle.

The rotation of the timing motor I03 through one complete cycle takes about six seconds. Switch III, which controls current to the starting winding I00, is closed for about four seconds. That is sufilcient time to bring the rotor of the drive motor I0 to synchronous speed at the maximum frequency of 400 cycles.

If it is desired to maintain the useful wattage employed by the motor I6 constant at all frequencies, an attenuating network may be inserted between points I45 and I46 of the pre-amplifier, and points I41 and I48 of the power amplifier, in the manner disclosed in applicants copending application, Serial No. 386,874, now Patent No. 2,364,644.

When the switch 80 is closed to start the synchronous motor I 6, current will flow from point 84 through points 86 and 81, through the motor running winding 88, through point 80, and returns to point 85. Current also fiows from point 81 to contact 8I. It is assumed that motor I6 is just being started, so that relay coil IOI will become energized for the duration of time necessary to provide current through the starting winding I00. When relay coil IN is energized, contact 8I is in dotted line position shown in Fig. 1, which means that current continues on the path through contact 8I in the dotted line position, through conductor strip 82, through switch 83, through one of the starting condensers 30, 81, 88, 88, through the starting winding I00, through point 80 and returns to point 85.

Points 85 and I50 are voltage between grounded. so that the points I50 and 86 is found equal to that between points 84 and 85. A slightly lower voltage exists between points I50 and II, and a still lower voltage between points I 50 and I52.

A condenser I53 is connected across the points I50 and I52. It is a well known action of condcnsers in alternating current circuits, that their impedance decreases with an increase in frequency. Therefore, if the frequency generated by the frequency control disc 22 is raised, the voltage between points I50 and I02 will be lowered, and

if the frequency is lowered, the voltage between 5 these points will be raised.

This condenser action is utilized by use of a diode rectifying vacuum tube I55, which allows negligible current to fiow through it in one direction only. The output from this tube at point I51 fiows to the pre-amplifier C, to control the amount of amplification of the pre-amplifier C. The voltage obtainable in this manner from point I00 is applied to a control grid of one of the amplifying tubes in the pro-amplifier C, and controls the output voltage in such a manner that the output voltage is proportional to its frequency. This automatic device regulates the overall amplification value of the preamplifier in such a manner that the output voltage is always the correct value for each operating frequency.

It is desirable to employ a slightly higher voltage on the drive motor I0 during the starting period. For that purpose there is provided a resistor I00 between points 00 and I5I, and a contact 0|, which when in the solid line position shown, short circuits this resistor I60. This contact II will always be in the solid line position when relay coil "I is not energized.

During part of the timing cycle, the relay coil dotted line position shown. with this contact II in the dotted line position, the percentage of voltage drop between points I00 and I02 will be lowered, and the ultimate control voltage applied along the output wire at the point I01 will be lowered. With a lower control voltage applied to the pre-amplifier from point I01, the actual output voltage will be slightly higher than it would be without the series resistor I00 connected in the circuit.

Upon de-energization of the relay coil "II, the contact 0I is moved to the solid line position, so that resistor I00 will be short-circulted, and the actual output voltage lowered.

The synchronous driving motor I0 is of the capacitor start ime. quiring a starting condenser in series with the starting winding I00. The purpose of this condenser is to obtain an amount of electrical shift of the phase, depending upon the frequency of the applied current and the capacity of said condenser.

With the wide range of frequencies employed in operating motor I 0, it has been found that the higher the frequency, the lesser the capacity required. For synchronous motor operation from 20 to 400 cycles, it has been found that four condensers, 00, 01, 80 and 00, of different capacity, are required to cover this range of frequencies.

For Instance, condenser 80 has proper capacity so for frequencies from about 20 to 100 cycles per second; condenser 01 for frequencies of about 100 to 200 cycles; condenser 00 for frequencies of about 200 to 300 cycles, and condenser 88 for frequencies of about 300 to 400 cycles.

The proper condenser is connected in series with the starting winding I00 by the same mechanism, which shifts the exciter lamp 28 and photo-electric cell 21 into registry with a selected row of the frequency control disc 22. For that purpom four terminal strips, I80, I8I, I82 and I83, connected to condensers 88, 81, 88 and 88, respectively, are mounted alongside of each other as shown in Fig. 2, while a conductor strip 82 is mounted below said terminal strips. Rigid with 70 thearmllsreapairofswitchblades 83am! I88 carrying brushes riding in electrical contact with the upper and lower strips, the proper condenser in series with'thestarting winding iilli.

To generate three phase alternating current, three identical frequency control discs would be mounted on the shaft of a single synchronous motor 25. Each disc would be provided with a light source and photocell combination, but with all three lamps and photo-electric cells mechanically tied together. If the light focused on the first control disc was just commencing to pass through the light transmitting section, and on to the first photo-electric cell, then the second control disc would be so arranged that it would be one-third of a cycle behind the first disc, and

and serving to connect the third disc would be one-third of a cycle behind the second disc. With three separate frequency control discs and three separate amplifier units, it is possible to generate three alternating currents whose phase relationship has been determined by the mechanical arrangement of the three frequency discs. To shift to another frequency, it would only be necessary to mechanically shift the three photo-electric cell combinations in registry with the desired annular frequency control row.

As many changes could be made in the above method and apparatus. and many apparently widely different embodiments of this invention could be made without departing from the scope of the claims, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. Method of controlling the speed of a synchronous driving motor having a capacitor starting winding and a running winding, which includes the steps of generating periodic current pulsations at a single, constant, preselected frequencv corres onding to the desired speed of the motor; amplifying said current to a value sufficient to operate said motor: delivering the amplified current initially to both the starting and running windings 'of the motor at the preselected frequency; automatically controlling the voltage of the amplified current in proportion to the preselected frequency; and automatically disconnecting the starting winding when the motor reaches a speed sufiicient to permit it to continue synchronous operation at the speed corresponding to the frequency of the amplified current.

2. Method of controlling the speed of a synchronous driving motor having a capacitor starting winding and a running winding, which includes the steps of interrupting a beam of light at regular and uniform intervals of a single, con stant, preselected frequency; generating an alternating current of said preselected frequency by the action of said light beam; amplifying said current to a value sufficient to operate said motor; delivering the amplified current initially to both the starting and running windings of the motor at the preselected frequency; automatically controlling the voltage of the amplified current in proportion to the preselected frequency; and automatically disconnecting the starting winding when thte motor reaches a speed suflicient to permit it to continue synchronous operation at the speed corresponding to the frequency of the amplified current.

3. In combination, a synchronous motor having a capacitor starting winding and a running winding; a generator for producing periodic current pulsations at a single, constant. preselected frequency corresponding to the desired speed of the motor; means to vary and select the single, constant frequency of current delivered by said generator; means to amplify said current to a value sufiicient to operate said motor; circuit means to deliver the amplifying current initially to both said starting and running windings of the said motor at .the preselected frequency; means to correlate the voltage of said amplified current in proportion to said preselected frequency; and circuit interrupting means to disconnect said starting winding'when said motor reaches a speed suillcient to permit it to continue synchronous operation at the speed corresponding to the frequency of said amplified current.

4. In combination, a synchronous driving motor having a capacitor starting winding and a running winding; a light reactive device for generating an alternating current; a light source arranged to project light on to said light reactive device; means for periodically interrupting the passage of light from said source to said device at a single, constant, preselected frequency to generate current in said device at a frequency corresponding to the desired speed of said motor; selector means to Vary the period of interruption of the passage of light from said source to said device in accordance with the single, constant, preselected desired speed of said motor; means for amplifying said current to a value sufllcient to operate said motor; circuit means to deliver the amplified current initially to both said starting and running windings of said motor at said preselected frequency; automatic circuit means for correlating the voltage of said amplified current in proportion to said preselected frequency; and automatic means for disconnecting said starting winding when said motor reaches a speed sufficient to permit it to continue synchronous operation at the speed corresponding to the frequency of said amplified current.

5. Means for producing an alternating current of different frequencies for operating a synchronous motor at different speeds, including a light source, alight reactive device for generating current, a screen between said light source and said light reactive device having a series of rows of spaced obstructions preventing the passage of light therethrough, the obstructions of each row being repeated at equal spaced intervals, different from those of the other rows, said screen bei movable across the path of light between said light source and said light reactive device and in a direction to bring and maintain a selected one of said rows of obstructions continuously in said light path, whereby current pulsations are generated in said device having a frequency depending on the frequency with which said obstructions pass through said light path, means for adjustably moving said screen relatively to said light reactive device and said light source to selectively bring any one of said rows of light varying obstructions into said light path, means for amplifying the current pulsations in said device toa value for motor operations, and delivering the amplified current to the motor to operate the latter at a fixed speed determined by the particular row of light obstructions which are in the pathoi the light beam.

6. An apparatus for controlling the speed of a synchronous motor comprising a light source,

means including a light reactive device for generating current pulsations. a disc between said light source and said light reactive device having aseriesoi' annular concentric rows of light varying obstructions, the obstructions of each row being repeated at equal spaced intervals, difierent from those of the other rows, means for rotating said disc at a uniform speed, means for relatively adjusting said screen with respect to said light reactive device and said light source to selectively bring any one 0! said rows of light varying obstructions into the path between said light source and said light reactive device, whereby th resulting light variations on said device generate current pulsations having a frequency depending on the frequency with which said ohstructions pass through said light path, means for amplifying the current pulsations to a value for motor operations, and means for delivering said amplified current pulsations of the selected frequency to said motor to operate said motor at the desired selected and corresponding speed.

7. An apparatus for controlling the speed of a synchronous motor comprising a light source, means including a light reactive'device for generating current, a disc between said light source and said light reactive device having a series of annular concentric rows of light obstructions, the obstructions of each row being repeated at equal uniformly spaced intervals, diflerent from those of the other rows, means for rotating said disc at a uniform speed, an indicating device, an op;

erating member, means responsive to the operation of said member for moving said indicating device into selected indicating position and for simultaneously andautomatically moving said light reactive device and said light source with respect to said screen to bring the row of light obstructions corresponding to the indicating position of said indicating device into the path of light between said light source and said light reactive device, whereby the resulting light variations on said light reactive device generate current pulsations having a frequency depending on the frequency with which said obstructions pass through said light path, means for amplifying the current pulsations to a value for motor operations, and means for delivering said selected amplified current pulsations to said motor to operate said motor at the desired selected speed.

ALFRED 1". CHOUINARD.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number 

